The 33-year-old Brantly, who is married and has two children, was talking with his doctors and working on his computer while being treated, said Samaritan's Purse spokeswoman Melissa Strickland. He is in stable and very serious condition, said Samaritan's Purse Vice President Ken Isaacs.

Dr. Kent Brantly is shown in this 2013 photo provided by JPS Health Network. A relief group official says Brantly is one of two American aid workers that have tested positive for the Ebola virus while working to combat an outbreak of the deadly disease at a hospital in Liberia. A spokesman said both Americans have been isolated and are under intensive treatment.(AP Photo/JPS Health Network)(Photo: AP)

Jan Brantly said her son had wanted to be a medical missionary from an early age. The Indianapolis Heritage Christian High School graduate began going on mission trips while young and has also worked in Uganda and Tanzania, she said.

Brantly graduated from the Indiana University School of Medicine in 2009, a school spokeswoman said. She said Brantly performed humanitarian work while still in medical school, working with impoverished, inner-city residents.

Brantly volunteered to fight Ebola in Africa soon after finishing his hospital residency in Texas.

He was being treated in the Monrovia hospital where he worked. "We are hopeful, but he is certainly not out of the woods yet," said Strickland. Early treatment improves a patient's chances of survival, and Brantly recognized his own symptoms and began receiving care immediately, she said.

There is no known cure for the highly contagious virus, which is one of the deadliest in the world. At least 1,201 people have been infected in Liberia, Sierra Leone and Guinea, according to the World Health Organization, and 672 have died. Besides 129 Liberian fatalities, 319 people have died in Guinea and 224 in Sierra Leone.

Medical staff are at the highest risk of contracting the virus because of their proximity to patients. Health workers attending to the patient should wear gloves, gowns and face visors to prevent contact between the patient's bodily fluids and their own mucus membranes, such as the eyes, mouth, nose and ears, said David Heymann, a professor at the London School of Hygiene and Tropical Medicine who has studied Ebola since 1976.

Strickland called Brantly "meticulous," saying that's why he was given the job. "We have every confidence that those protocols were not breached," she said. The group is investigating how Brantly may have been exposed.